Electronic valve amplifiers



Nov. 10, 1959 1.. B. MULLETT 2,912,620

ELECTRONIC VALVE AMPLIFIERS Filed Dec. 14, 1956 FIG.I.

United States Patent ELECTRONIC VALVE AMPLIFIERS Leslie Baden 'Mullett',Abingdon, England, assignor to The United Kingdom Atomic EnergyAuthority of Patents Branch, London, England Application December 14,1956, Serial Narz'sns-i TClaims. totals-41' This invention relates toelect-ronicvalve amplifiers and is particularly concerned with cathodestructures for such 2,912,620 Patented Nov. 10, 195? ice ' andamplifiers capable of producing a large current of stably bunchedelectrons.

The invention is based on the mult pactor effect which is a phenomenonassociated with surfaces whose secondary emission coefficient can(within a given primary Consider two parup extremely rapidly. Thephenomenon occurs over :9 a range of phase angle (the angle during acycle between zero field and the appearance of an electron at a surface)determined by the distance between the plates, the strength of the R.F.field and the secondaryemission characteris-' tics of the surfaces.

A multipactor oscillator making use of the above effect is describedbyBaker'in US. Patent No. 2,674,694. A

secondary emission electron multiplier utilizing the effect is alsoknown, the electrons being collected by a probe entering the spacebetween the plates.

According to the presentinvention in a cathode structure comprising twoparallel plates adapted to support a R.F. electric field between themnormal to th ir surfaces and to generate electrons by the multipactoreffect, one of said plates is perforated such that an anode placedbeyond the perforated plate collects those bunches of electrons whichpass through the perforations.

An amplifier valve in accordance with the, invention may comprise such acathode structure, an associated anode structure, means for feeding R.F.powerfinto'the space between the plates (the multipactor space) andmeans for extracting R.F. power from the space between the cathode andanode structures (the electron interaction space). Y Y

The spaces into which the R.F. power is fed in and extracted may be inthe form of resonant or travelling wave cavities. I

In order that large quantities of R.F. power may be generated, the mostadvantageous construction is one allowing the inclusion of a large areaof cathode. One such construction comprises a double coaxial line, theinner line providing the multipactor space and having a perforated outerconductor constituting the inner conductor of the outer line whereinelectron interaction takes place, a D.C. voltage being applied to theouter sheath.

Another construction involving travelling wave cavities may comprise adouble ridge-loaded waveguide, the septum being perforated and definingthe multipactor space in relation to one of the ridges and theinteraction space in relation 'to the other ridge.

Fig. 3 is a diagram similar to Fig. 2 wherein adouble rectangularwaveguide provides said spaces.

Fig. 4 is a cross section of Fig. :3.

In Fig. 1 two B 10 cm. wavelength resonator cavities 1 and 2 are dividedby a septum 3 having a central .circular perforated ,area 4..overlying acylindrical cathode ,postS. The undersurface .(in the diagram) of theperforated area 4 and the upper surface of the post 5 con- Sti lt catode and define the multipactor gap. (space) and are of oxidisedberyllium-copperto provide high secondary emission coeflicients, i.e.1.4. I i

The input resonator 1 is designed to give only one mode of mu t pa t.efi themu tipactor g p length .being about .12 cm'so that an energyinthe region of 500 volts is given to electrons having a halfcycle'transit-time. The post 5 is 1 cm. diameterand the length of theresonator is 1.2 cm. The arrangement limits the multipactor effect toaregion where the ,fieldis appreciably constant and providesa reasonableQ and feed system. The latter consists of a loop 6 extending frombetween the inner andouter conductors of a coaxialline 17.

The area 4 is perforated withclosely spaced 2 mm. diam. holes to give atransmission ratio -i.e. area of ;holes/ total .area of more than .5

The resonator 2 is provided with an anode 7 opposite the perforated area4 and with an output coupling.8 to a coaxial line 9.

In operation, on applying pulses to the input line -17, steadywell-shaped electron current pulses equivalent to .25 amp/cm. areobtained with a mean energy of 300400 volts.

The current obtained for a given transmission ratio of the area 4 isfound to be insensitive to contamination of the cathode surfacesor tothe precise'degree of vacuum.

The above described constructionisrnost advantageous for P d i g g p wQnjlo g .wave engths of the order of cm. For examp efat that wavelengtha 50 kv. valve could have a diameter of 50-70 cm. and a total'resonatorlength of about 78 cm, giving a current of the order of 200 amperes.

Referring to Fig. 2, the valve takes the form of a length of doublecoaxial line. The cathode 5a is a cylindrical enlargement of theinnermost conductor ,10 and the perforated area 4:: is formed in theouter conductor 3a of the inner coaxial line (the inner conductor of theouter coaxial line). The multipactor space defined by the area 4a andthe cathode 5a is thus annular. The outermost conductor 7b constitutesthe anode and is supplied with a D.C. voltage. R.F. power is fed to theinner space and amplified R.F. power withdrawn from the outer space byloops as in the above described embodiment.

In Figs. 3 and 4 is shown a double rectangular waveguide comprisingchannels In and 1b separated by a 'field and occurs when the ratio is1.38.

- 3 gain in the other) feed back circuits (not shown) are provided onboth channels.

In arriving at suitable parameters for such a valve two conditions foran electron in the interaction space are considered-one in'which thetransit time is very short and one in which it is comparable with half acycle.

If the transit time is very short it can be shown that the electronvelocity will remain at its value at injection .and this low velocitycombined with a short transit time calls for an extremely smallinteraction space.

It is, however, desirable to allow the electrons to speed up immediatelyafter injection and to allow a con siderable variation of R.F. fieldduring the transit time.

It can be shown that there is an optimum transit time determined by theratio E /E where E is the maximum amplitude of the R.F. electric fieldand E is the DC. Also the width l of the interaction space in cm. isrelated to E for a wavelength by the expression l/A :E 2.474

Where x is in cm. and E in volts/cm. and if V is the applied DC. voltagethen.

Also, it is found that if the dimension l is scaled with wavelength, theapplied voltage V, the quantity E and hence the power flux W are allconstants. The following table shows values of valve parameters based onthese calculations, where W is the power flux in an H rectangularwaveguide (channel 1b) of dimensions 1 and EM (kv.) Em (kv.) l/M v (kv.)W (mw,)

500 690 01237 6.365 2. 045 636 877 0157a 10. 0 4. 201 750 1, 035 .o1s5s13.92 6. 904

The fraction of the circulating power which is available as power outputis to some extent controlled by the allowable variation of field alongthe length of the valve, but is ultimately determined by the currentsince the DC. will have been fixed.

For a given electron current the driving power is determined by theenergy given to the electrons in multipactoring. If each electronacquires about 500 electron volts in a half cycle transit, the, energyrequired to proelectrons by the multipactor effect, one of said platesbeing perforated, an anode placed beyond the perforated plate to collectthose bunches of electrons which pass through the perforations, meansfor feeding R.F. power into the multipactor space between the plates andmeans for extracting R.F. power from the electron interaction spacebetween the cathode and anode structures.

2. An electronic valve amplifier as claimed in claim 1 comprising adouble coaxial line, the inner line providing the multipactor space andhaving a perforated outer conductor constituting the inner conductor ofthe outer line wherein electron interaction takes place.

3. An electronic valve amplifier as claimed in claim 1 comprising adouble ridge-loaded waveguide, the septum being perforated and definingthe multipactor space in relation to one of the ridges and theinteraction space in relation to the other ridge.

4. An electronic valve amplifier as claimed in claim 1 wherein saidmultipactor and electron interaction spaces are resonant cavities.

5-. An amplifier comprising in combination a cathode structureconsisting of two parallel spaced members defining a multipactor space,one said member being perforated, an anode structure spaced from thesaid cathode and defining therewith an electron interaction space, meansincluding a resonator cavity for feeding R.F. power into the multipactorspace, and means including a second resonator cavity for deriving R.F.power at a higher level from the electron interaction space, the facingsurfaces of the said two parallel spaced members having secondaryelectron emission co-efiicients greater than 1.4. p

6. The invention as set forth in claim 5 wherein the said members areplanar.

7. An electron discharge device comprising three elongated cylindricalconductors arranged in coaxial relationship, the middle conductor beingspaced from the inner and the outer conductors, a portion of the innerconductor'being of larger diameter than the remainder thereof anddefining together with the opposed surface of the middle conductor a twoelement cathode structure, the said opposed surface being perforated, ananode structure comprising the portion of the outer conductor opposed tothe said portion of the inner conductor, means to feed R.F. power intothe spacing between the cathode elements, and means to derive R.F. powerat a higher level from the spacing between the outer cathode element andthe anode.

References Cited in the file of this patent UNITED STATES PATENTS2,147,934 Snyder Feb. 21, 1939 2,189,358 Farnsworth Feb. 6, 19402,278,210 Morton Mar. 31, 1942 2,295,396 George Sept. 8, 1942 2,416,303Parker Feb. 25, 1947 I 2,674,694 Baker Apr. 6, 1954

